1. Field of the Invention
The present invention relates to a thin film field effect transistor and a display using the same. Particularly, it relates to a thin film field effect transistor in which an amorphous oxide semiconductor is used for an active layer, and a display using the same.
2. Description of the Related Art
In recent years, flat panel displays (FPDs) have been put to practical use, due to the progress made in liquid crystal and electroluminescence (EL) technologies, etc. Especially, an organic electroluminescence element (hereinafter referred to as an “organic EL element” in some cases) formed using a thin film material which emits light by excitation due to application of electric current can provide light emission of high brightness at low voltage, and thus is expected to achieve reduction in device thickness, weight, and size, and power saving, etc. in wide ranging applications including mobile phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination.
These FPDs are driven by an active matrix circuit including thin film field effect transistors each using, as an active layer, an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate. (In the description below, the thin film field effect transistor is sometimes referred to as a “thin film transistor” or “TFT”.)
On the other hand, to make the FPD thinner, lighter, and more resistant to breakage, attempts are being made to use a resin substrate which is light in weight and flexible instead of the glass substrate.
However, fabrication of the transistors using the silicon thin films described above requires a thermal treatment process at a relatively high temperature, and it is difficult to form the transistors directly on a resin substrate which is generally low in heat resistance.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2006-121029, a MOSFET (metal-oxide semiconductor field effect transistor) that reduces a driving voltage of a transistor that uses a silicon thin film is disclosed, and a configuration that uses indium tin oxide (ITO), tin oxide, zinc oxide or the like as a semiconductor material of an active layer and uses a dielectric material having a large dielectric constant in a gate insulating layer is disclosed. It is disclosed that ITO, tin oxide, zinc oxide or the like is a crystalline metal oxide and has a carrier concentration of substantially 1×1019 cm−3. In the case of an active layer made of the crystalline metal oxide, in order to obtain desired semiconductor characteristics, after film formation by sputtering, a high temperature heat treatment step such as post-annealing at 300° C. for 15 minutes is necessary in order to control the crystallization. Accordingly, such an active layer is difficult to form directly on a resin substrate that is poor in heat resistance.
JP-A No. 2000-124456 discloses a TFT using a two-layer structure of a channel layer comprising a thin film formed from amorphous silicon or the like and an offset layer comprising a thin silicon carbide film or the like as a liquid crystal screen control TFT. However, while the TFT can be utilized as a liquid crystal screen control TFT using a glass substrate, it is difficult to fabricate the TFT on a flexible resin substrate since the flexible resin substrate intrinsically involves the problem of heat resistance described above.
An amorphous oxide such as an In—Ga—Zn—O-based amorphous oxide can form a film at low temperatures, and, accordingly, has been attracting attention as a material capable of forming a film at room temperature on a plastic film. However, when an amorphous oxide semiconductor is used in an active layer of a TFT, an OFF current is high, and accordingly, there is a problem in that an ON/OFF ratio is low. Further, improvement in stability and reliability such as with respect to occurrence of hysteresis or change in electric characteristics of the TFT over time has been desired.
As means for solving the problems described above, a configuration in which an amorphous oxide insulating film containing In—M—Zn (in which M is at least one element selected from Ga, Al, Fe, Sn, Mg, Ca, Si or Ge) as a main constituent element and having an electric resistance value of 1011 Ω·cm or more is disposed as a resistance layer between an active layer and a gate insulating layer is disclosed. Further, there is disclosure of using an amorphous oxide semiconductor containing In—M—Zn (in which M is at least one element selected from Ga, Al, Fe, Sn, Mg, Ca, Si, or Ge) as a main constituent element and having an electric resistance value of less than 1010 Ω·cm as an active layer which constitutes a channel, thereby making the band gap of the amorphous oxide insulating film greater than the band gap of the amorphous oxide semiconductor layer (see, for example, JP-A No. 2007-73701).
It is disclosed that, when a carrier concentration of an amorphous oxide semiconductor is reduced to, for example, less than 1018 cm−3, a TFT operates, that when the carrier concentration is less than 1016 cm−3, a TFT having excellent ON/OFF ratio is obtained, and that, in order to impart more excellent low OFF current characteristics, the carrier concentration is preferably reduced to less than 1016 cm−3. As a method of forming the amorphous oxide semiconductor, there has been disclosed a method of forming an amorphous oxide semiconductor layer by integrating a plurality of oxide layers in which the composition and elements of each of the layers are different from each other, and melting and mixing metal ingredients of the respective layers with each other during the forming process (see, for example, JP-A No. 2007-73704). Further, JP-A No. 2007-123702 discloses using an oxide semiconductor as an active layer formed by integrating an oxide semiconductor as a base material, and an oxide layer-spacer material having oxygen atoms at a film thickness that is less than a film thickness that causes a tunnel effect.
However, in TFTs supplied for practical use, in addition to low OFF current and high ON/OFF ratio, it is required that the characteristics do not vary even under continuous driving, and that stable performance is exhibited even when conditions such as a temperature or humidity in an operating environment vary. That is, there still remain many problems to be overcome.